This invention relates to a hitch control system, and in particular to a hitch control system which is responsive to wheel slip.
There are two basic types of known hitch control systems which respond to sensed wheel slip. A system of the first type is described in U.S. Pat. No. 4,518,044, issued May 21, 1985 to Wiegardt et al., and assigned to the assignee of this application. In this system the hitch control signal includes an offset which is proportional to the amount of slip error (defined by difference between desired slip and actual slip)--the hitch control signal is an arithmetic combination of wheel slip error with other error signals such as draft force error and/or position error. The system described in the '044 patent includes an activity control for adjusting a gain value which multiplies not only slip error, but the combination of slip error, draft force error, engine speed error and position error. The system described in the '044 patent also includes an operator-controlled, rate-selecting switch for selecting slow and rapid rates of implement movement.
A system of the second type is described in U.S. Pat. No. 5,911,769, issued Jun. 15, 1999 to Orbach et al. The system of the '769 patent generates a slip signal based upon the apparent speed and the ground speed, and has a first operational mode which is operative when the slip signal is below a slip limit and second operational mode which is operational when the slip signal is above the slip limit. In the first mode, the system generates a hitch actuator control signal based upon the difference between the first or second operating parameter (draft force or position) and a reference value. In the second mode, the system can override control based on this difference by using a control signal based upon the difference between the slip signal and a slip reference value. The '769 system also includes a three-position rocker switch movable between OFF, ON and SET positions and which is used for turning a slip control function on and off and for setting a slip limit value.
Both types of system operate satisfactorily in high slip conditions, or when minimum depth variation is desired. However, when operating in light soils in rolling terrain, such systems may not control depth with sufficient accuracy, and they may operate too aggressively and cause overshoot of the hitch. Neither of these systems has an operator control solely for adjustment of the slip response of the system.